The invention relates to a process for stabilizing a solution of a leuco dye in an organic solvent, to the correspondingly stabilized solution and to a radiation-polymerizable mixture containing a polymeric binder, a polymerizable compound, a radiation-activatable polymerization initiator and a leuco dye.
Mixtures of the above-indicated composition are known in the art. The addition of a leuco dye has the effect that, after exposure, the mixture exhibits a visible contrast between exposed and unexposed areas, since leuco dyes are oxidized during or after exposure to form the corresponding dyes. Mixtures of this kind are described, for example in DE-A 31 31 448 and in EP-A 230 941.
Due to the fact that leuco dyes are slowly oxidized even in the dark, these mixtures tend to discolor, i.e., they gradually assume a blueish shade, even during storage in the dark. This tendency is increased when photooxidizing agents, for example, trihalomethyl compounds which split off halogen radicals upon exposure, are added to obtain an intensification of contrast. A number of stabilizers, for example, sulfur compounds, phenols and other reducing agents have therefore been recommended for photopolymerizable mixtures and materials of this kind.
DE-A 35 34 527 describes thiuramdisulfide for use as a stabilizer. Similarly to thiourea, thiuramdisulfide is a moderate stabilizing agent. These compounds have the disadvantage of reacting with copper surfaces, which gives rise to annealing tints, staining upon developing and problems in a subsequent electro-plating process. In addition, photosensitivity is reduced by thiuramdisulfides.
U.S. Pat. No. 3,042,575 mentions stabilizers comprising phenols, such as resorcinol or hydroquinone. In most cases, however, the effect of these compounds is inadequate.
DE-A 29 41 846 describes 2,4-dihydroxybenzaldoxime as a reducing stabilizer, which has a better stabilizing action. But even when this compound is added, shelf life is still insufficient and, in addition, the adhesion of the layer to copper is impaired.
DE-A 37 35 088 describes stabilization of mixtures of this kind by the addition of an epoxy compound. This addition is very effective and prevents discoloration of the components of the solid photopolymerizable layer, even upon prolonged storage.
It has now been found that oxidation of a leuco dye and, consequently, discoloration of a mixture which has been stabilized as described above still occurs, when the mixture is kept for a certain period in the form of a solution in an organic vent--which is often inevitable prior to the coating of a support. A particularly serious discoloration of the coating solution in the absence of light occurs in those cases in which the solution contains a halomethyl compound. This discoloration is, however, observed in leuco-dye solutions in general, i.e. even in solutions which only contain the leuco dye and no further dissolved components.